ESTRO 36 Abstract Book

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S523 ESTRO 36
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Conclusion
The application of mathematical models describing the
significant dependences of MOSFET TN-502RDM on their
measurements results in an accuracy increase besides an
improvement in precision. However, IVD implementation
in HDR prostate BT treatments as a possibility of real-time
decision making related to an error detection needs a
retrospective evaluation of a larger sample data to define
correctly these error detection thresholds.
PO-0944 Dosimetric influence produced by the
presence of an air gap between the skin and the
freiburg flap
M. Fernandez Montes1, S. Ruíz Arrebola1, R. Fabregat
orrás 1, E. Rodríguez Serafín1, J.A. Vázquez Rodríguez1,
M.T. Pacheco Baldor1, N. Ferreiros Vázquez1, M.A.
Mendiguren Santiago1, J.I. Raba Díez1, M.M. Fernández
Macho1, J.T. Anchuelo Latorre2, M. Ferri Molina2, A.
García Blanco2, I. Díaz de Cerio2, M.A. Cobo Belmonte2,
A. Kannemann2, J. Andreescu Yagüe2, M. Arangüena
Peñacoba2, N. Sierrasesumaga Martín2, D. Guirado
llorente 3, I. Bernat Piña 4, P.J. Prada Gómez2
1 Hospital Universitario Marqués de Valdecilla,
RADIOPHYSICS, Santander, Spain
2 Hospital Universitario Marqués de Valdecilla, Radiation
ONCOLOGY, Santander, Spain
3 Hospital Universitario San Cecilio, Radiophysics,
Granada, Spain
4 Hospital Universitario Marqués de Valdecilla, Medical
Oncology, Santander, Spain
Purpose or Objective
Surface applicators were proposed as a way to treat
superficial lesions with HDR brachytherapy. The Freiburg
Flap (FF) is an applicator used in this type of treatment
that has limited flexibility, so that in certain situations it
is not perfectly adapted to the surface treatment. The
purpose of this study is to quantify the discrepancy in the
TPS dose calculation produced by unsuitable positioning of
the applicator, as opposed to the ideal situation, when the
applicator is perfectly adapted to the patient's skin leaving
no air gap.
Material and Methods
Nucletron FF, is an applicator comprising of silicone
spheres attached to each other, 1 cm in diameter,
arranged in parallel rows, capable of adapting to the
surface to be treated.
The TPS Brachy Nucletron Oncentra (Elekta, v-4.5.2) was
used for dose calculation using an 192Ir radiation source and
radiochromic film (Gafchromic EBT3) have been used for
dose measures which were subsequently analyzed with
ImageJ
To quantify the discrepancy between the TPS dose
calculation and the real administrated dose when
adaptation to the surface is not suitable, the experimental
setup designed shown in figure 1 was made, where we can
distinguish two regions between flap and film. In one
region, the film is at a distance of 5 mm from the
applicator, and in the other region at a distance of 7 and
9 mm (5mm of PMMA plus 2 or 4mm air gap
respectively).Two different treatment plans have been
designed, in the first one the source stops in the center of
the spheres and in the other one at the edge, to compare
the difference between dwell positions. The dwell times
are set to get the dose distribution as uniform as possible,
prescribing 6 Gy at a depth of 5 mm.
Results
Results obtained are shown in table 1. Underdosage is
observed, produced by air layers, ranging from 4.8% to
10.8% when dwell positions are at the center of the
spheres, and from 6.2% to 11.8% when dwell positions are
at the edge of the spheres, with 2 and 4 mm air gap
respectively.
Conclusion
In view of the results obtained, it can be concluded that
several layers of air between the applicator flap and the
skin can lead to considerable variation in dosimetry, which
may involve the loss of effectiveness of treatments with
this type of applicators. Thus, utmost care is required
during the placement of the flap to minimize the error due
to the air gap, therefore avoiding an underdosage in the
volume to be treated.
PO-0945 Pretreatment verification for brachytherapy
G. Fonseca 1, M. Podesta1, M. Bellezzo1, B. Reniers2, F.
Verhaegen1
1Maastro Clinic, Physics, Maastricht, The Netherlands
2University of Hasselt, NuTeC, Hasselt, Belgium
Purpose or Objective
Individual plan QA is not performed in brachytherapy
mostly due to the large uncertainty associated with dose
measurements. Traditional setups require precise and
accurate positioning, and therefore usually laborious
procedures to detect anything other than large
discrepancies with an unclear distinction between source
or detector mispositioning. This study evaluates the use of
an Electronic Portal Imaging (EPID) to verify the treatment
plan.
Material and Methods
The EPID panel response was characterized with an High
Dose Rate (HDR) Ir-192 source. A robotic arm was
employed for positioning within a water tank (Figure 1a)
assuring 0.2 mm accuracy during the calibration, which
covered a clinically relevant range for the distance
between the source and the panel (from 6 up to 25 cm).
Experiments were performed with an acquisition rate of
6.7 fps for a single catheter and for a gynecological